Janus kinase (JAK) is a family of intracellular non-receptor tyrosine kinases, ranging from 120-140 kDa, that transduce cytokine-mediated signals via the JAK-STAT pathway. The JAK family plays a role in the cytokine-dependent regulation of proliferation and function of cells involved in immune response. Currently, there are four known mammalian JAK family members: JAK1, JAK2, JAK3 and TYK2.
JAK1, JAK2 and TYK2 are ubiquitously expressed whereas JAK3 is expressed in the myeloid and lymphoid lineages. The JAK family members are non-receptor tyrosine kinases that associate with many hematopoietin cytokines, receptor tyrosine kinases and GPCR's. JAK1−/− mice were found to be developmentally similar to the JAK1+/+ although they weighed 40% less than the wild-type and failed to nurse at birth. These pups were not viable and died within 24 hours of birth (Meraz et al Cell, 1998, 373-383). JAK1 deficiency led to reduced number of thymocytes, pre-B cells and mature T and B lymphocytes. TYK2(−/−) mice, on the other hand, are viable, demonstrating subtle defects in their response to IFN-α/β and IL-10 and profound defects to the response of IL-12 and LPS.
The breast cancer susceptibility protein (BRCA1) acts as a tumor suppressor and contributes to cell proliferation, cycle regulation, as well as DNA damage and repair. BRCA1 (−/−) mice develop normally but die by 7.5 days post embryo suggesting a key role of BRCA1 for development. Mice in which the BRCA1 protein was overexpressed led to inhibition of cell growth and sensitized cells to cytotoxic reagents. In the human prostate cancer cell line Du-145 (Gao FEBS Letters 2001, 488, 179-184), enhanced expression of BRCA1 was found to correlate with constitutive activation of STAT3 as well as activation of JAK1 and JAK2. Moreover, antisense oligonucleotides selective for STAT3 led to significant inhibition of cell proliferation and apoptosis in Du-145 cells. This data supports the potential utility of JAK1 and JAK2 inhibitors in the treatment of prostate cancer.
Campbell et al (Journal of Biological Chemistry 1997, 272, 2591-2594) as reported that STAT3 is constitutively activated v-Src transformed cells. To test whether STAT3 activation resulted via signaling through the JAK-STAT pathway, three fibroblast cell lines (NIH3T3, Balb/c, and 3Y1) were transformed with v-Src. The level of JAK1 phosphorylation in NIH3T3 cells was markedly increased in cells overexpressed with v-Src or mutant c-Src (Y527F) compared to those in the less transforming c-Src. This result correlated with increased JAK1 enzymatic activity. Similar results were observed with JAK2 albeit to a lesser extent. These results are consistent with constitutive activation of JAK1 and possibly JAK2 which contribute to the hyperactivation of STAT3 in Src-transformed cells.
Asthma is a disease that is increasing in prevalence and results in “airway obstruction, airway hyperresponsiveness, and airway inflammation and remodeling” (Pernis The Journal of Clinical Investigation 2002, 109, 1279-1283). A common cause is the inappropriate immune responses to environmental antigens usually involving CD4+ T helper cells (TH2) which are triggered from cytokines IL-4, IL-5, IL-6, IL-10, and IL-13 which signal through JAK1/JAK3-STAT6 pathway. Th1 cells are thought to be involved with the “delayed-type hypersensitivity responses” which secrete IL-2, IFN-γ, and TNF-β and signal through the JAK2/TYK2-STAT4 pathway. STAT6 (−/−) mice were protected from AHR when challenged with environmental antigens and showed no increase in IgE levels or the quantity of mucous containing cells.
JAK2 is a cytoplasmic protein-tyrosine kinase that catalyzes the transfer of the gamma-phosphate group of adenosine triphosphate to the hydroxyl groups of specific tyrosine residues in signal transduction molecules. JAK2 mediates signaling downstream of cytokine receptors after ligand-induced autophosphorylation of both receptor and enzyme. The main downstream effectors of JAK2 are a family of transcription factors known as signal transducers and activators of transcription (STAT) proteins. Studies have disclosed an association between an activating JAK2 mutation (JAK2V617F) and myeloproliferative disorders. The myeloproliferative disorders, a subgroup of myeloid malignancies, are clonal stem cell diseases characterized by an expansion of morphologically mature granulocyte, erythroid, megakaryocyte, or monocyte lineage cells. Myeloproliferative disorders (MPD) include polycythemia vera (PV), essential thrombocythemia (ET), myeloid metaplasia with myelofibrosis (MMM), chronic myelogenous leukemia (CML), chronic myelomonocytic leukemia (CMML), hypereosinophilic syndrome (HES), juvenile myelomonocytic leukemia (JMML) and systemic mast cell disease (SMCD). It has been suggested that abnormalities in signal transduction mechanisms, including constitutive activation of protein tyrosine kinases, initiate MPD.
JAK3 associates with the common gamma chain of the extracellular receptors for the following interleukins: IL-2, IL-4, IL-7, IL-9 and IL-15. A JAK3 deficiency is associated with an immune compromised (SCID) phenotype in both rodents and humans. The SCID phenotype of JAK3−/− mammals and the lymphoid cell specific expression of JAK3 are two favorable attributes of a target for an immune suppressant. Data suggests that inhibitors of JAK3 could impede T-cell activation and prevent rejection of grafts following transplant surgery, or to provide therapeutic benefit to patients suffering autoimmune disorders.